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Pathogenesis of Neoplasms Quiz

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A 6-year-old child has bilateral retinoblastoma (a cancer of the retina in both eyes) and a strong family history of the disease. Which statement best explains the molecular basis of this inherited cancer susceptibility?

A Loss of a DNA repair gene directly causes a retinal tumour by accelerating the cell cycle
B A single activating mutation in a proto-oncogene is enough to cause cancer because it creates a dominant gain of function
C Overexpression of growth factor receptors needs both copies of the gene to be mutated
D Inherited cancer syndromes require a germline mutation in an oncogene that is present in every cell
E Inheriting one faulty tumour suppressor gene copy leaves cells needing a single further hit to lose all gene function ✓

The answer is E: Inheriting one faulty tumour suppressor gene copy leaves cells needing a single further hit to lose all gene function

Explanation

Why E is correct

See Intrinsic Factors. Option E is correct because retinoblastoma illustrates the two-hit hypothesis proposed by Knudson. A tumour suppressor gene such as RB1 acts as a brake on cell division, and a cell normally has two copies, so both must be lost before the brake fails. In the inherited form, the child inherits one faulty copy through the germline (the egg or sperm), so every cell already carries the first hit. Only one further somatic mutation (a mutation acquired in a body cell) in any retinal cell is then enough to remove the second copy, which is why the tumours often appear early in life, in both eyes and at several sites. In the non-inherited form, two separate somatic hits are needed in the same cell, so the cancer is rarer, later and usually affects one eye. Memory aid: inherited means the first hit is free, so only one more is needed.

Two-hit hypothesis: both copies of a tumour suppressor gene such as RB1 must be lost before the cell escapes control.
Inherited form: the first hit is a germline mutation in every cell, so one somatic hit is enough.
Non-inherited form: two somatic hits are needed in the same cell, so the cancer is rarer and later, usually in one eye.

Why the other options are incorrect

A. Loss of a DNA repair gene directly causes a retinal tumour by accelerating the cell cycle: A DNA repair defect would increase the mutation rate, but retinoblastoma is caused by loss of the RB1 tumour suppressor, so replacing it with accelerated cell cycling is incorrect.
B. A single activating mutation in a proto-oncogene is enough to cause cancer because it creates a dominant gain of function: This is a true oncogene mechanism, but the patient has inherited retinoblastoma caused by loss of a tumour suppressor gene, not an activated proto-oncogene, so it does not explain the susceptibility.
C. Overexpression of growth factor receptors needs both copies of the gene to be mutated: Overexpression of a receptor is a gain of function, which usually needs only one altered copy. It does not explain the pattern of an inherited loss of a tumour suppressor.
D. Inherited cancer syndromes require a germline mutation in an oncogene that is present in every cell: This names a germline oncogene mutation, but the inherited susceptibility in retinoblastoma comes from a tumour suppressor gene, so the option does not fit the clinical picture.

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1
Which feature is specific to malignant neoplasms and is not seen in benign tumours?
2
Which behavioural factor most directly increases cancer risk by acting as an avoidable environmental agent that causes DNA mutation?

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